1. Field of the Invention
The present invention relates to the field of combustion technology. It relates to a method for burning a liquid fuel in a combustion system.
2. Description of the Related Art
Thermoacoustic oscillations represent a danger to all types of combustion applications. They lead to high-amplitude pressure fluctuations, to constriction of the operational range, and can increase undesirable emissions. This affects, in particular, combustion systems with little acoustic damping, such as those used in gas turbines. Active control of the combustion oscillations may be required to guarantee high performance with regard to pulsations and emissions over a wide operating range.
Various techniques for controlling and suppressing combustion instabilities by means of an active control system have already been proposed, in which, using either an open or a closed control loop, the supply of fuel and/or combustion air to the burner or to the burners is controlled or modulated in a defined manner. A prior, not previously published application from the applicant relates, for example, to active control of the instabilities in a premixing burner and is illustrated, for example, in FIG. 1 of EP-B1-0 321 809. In such a premixing burner, the fuel flows in the two outer fuel lines (8, 9 in FIG. 1 of EP-B1-0 321 809) are modulated asymmetrically in an open loop at frequencies between 0.3 Hz and 5 kHz, preferably between 5 Hz and 200 Hz. The modulation process is carried out with the aid of two fuel valves which are inserted in the fuel lines.
A disadvantage with the use of mechanically moving, electrically driven fuel valves is that they have mechanically moving parts which not only limit the range of the modulation frequency but, at the modulation frequencies which are used, are also subject to increased wear and whose functional reliability is subject to limitations. Another disadvantage is the relatively high power required by the valves themselves, which makes considerable additional circuit measures necessary.
The object of the invention is to provide a combustion method and a combustion system which effectively suppress thermoacoustic combustion instabilities in a simple and effective manner and without any mechanically moving parts by modulation over a wide frequency range.
The essence of the invention is for the liquid fuel to be injected with electrostatic assistance. The application of an electrical voltage to the fuel allows the spray angle and the range of the injected fuel spray, the atomization and the resultant droplet size distribution, with the vaporization and distribution of the fuel to be influenced. If this influencing process is carried out in accordance with a predetermined, selected time function, the combustion flame and thus the instantaneous rate at which heat is released can be modulated appropriately. Since the instantaneous rate at which heat is produced represents the most important single factor in the production and suppression of thermoacoustic combustion instabilities, the instabilities can effectively be suppressed by suitable selection of the time function and other modulation variables.
According to one preferred embodiment of the method according to the invention, a high voltage of several kV is used for modulation and is, in particular, a pulsating DC voltage, with a periodic function being used as the time function.
In this case, the high voltage can on the one hand be modulated in an open control loop. This is justified in particular if the instabilities in the combustion chamber have a response which is constant over time and can be suppressed or attenuated by selection of a suitable fixed time function.
If, on the other hand, the instabilities vary with time or as a function of various changing operating parameters, it is expedient and advantageous to monitor the hydrodynamic instabilities continuously by measuring suitable variables, in particular the instantaneous rate at which heat is released or the pressure pulsations, and to carry out the modulation process in a closed control loop, including the measured variables.
The combustion system according to the invention, which comprises a combustion chamber and at least one injection nozzle through which liquid fuel is injected into the combustion chamber, is distinguished in that first means are provided in order to apply an electrical voltage to the liquid fuel during the injection process.
One preferred embodiment of the combustion system according to the invention is distinguished in that the injection nozzle comprises a nozzle bore and a fuel channel which leads to the nozzle bore, in that the fuel in the fuel channel has the modulating electrical voltage applied to it, in that the injection nozzle comprises a nozzle body in which the nozzle bore is arranged, in that the fuel channel is formed between the nozzle body and an insert which is inserted with a gap into a recess in the nozzle body and is electrically insulated from the nozzle body, and in that the modulating electrical voltage is applied between the nozzle body and the insert.